This invention relates to fluoroalkene-nucleophile adduct formation for removal, immobilization, or quantification of fluoroalkenes from fluids.
Perfluoroalkane fluids have many industrial uses, such as coolants for electronic devices (e.g., supercomputers) and as heat transfer media in vapor-phase soldering processes. However, upon transient heating many of these perfluorinated liquids at high temperatures, toxic impurities may form, such as certain fluoroalkenes, for example perfluoroisobutene (PFIB). These impurities may be hazardous to persons handling the liquid or operating equipment containing the contaminated liquid. Analytical procedures for the identification and quantification of the highly volatile low molecular weight fluorocarbons generally require chromatographic separation and reference standards for calibration. The more toxic perfluoroolefins such as PFIB are not readily available to be used as reference standards and transportation is a serious problem. Marhevka et al., Anal. Chem., 1982, 54, 2607-2610, describe a method to generate a reference standard and suggest the use of an analytical surrogate, perfluorocyclopentene (PFCP), for calibration purposes.
Various methods have been suggested for reducing the hazard of PFIB exposure of operators of equipment that might inadvertently produce PFIB (Turbini, L. J., Zado, F. M., xe2x80x9cChemical and Environmental Aspects of Condensation Reflow Solderingxe2x80x9d, Electronic Packaging and Production, January, 1980, 49-59 and xe2x80x9cFluorinert Liquidsxe2x80x9d, 3M Publication No. 98-0211-4411-2(78.2)R1 XY, June 1988). Some of these methods include techniques of operating and maintaining vapor-phase soldering equipment to avoid localized super-heating of perfluorinated liquids, thus reducing the amount of PFIB produced, and standards of designing work areas to provide sufficient ventilation to maintain PFIB levels at less than hazardous levels.
U.S. Defensive Publication T983,009 (June, 1979) describes a method of converting PFIB in a mixture of fluorine-containing compounds into a relatively nontoxic ether by contacting the mixture with a solution of methanol and a selected hydrogen halide. While this method does produce products which are generally less toxic than PFIB, it has disadvantages, including 1) being complex to perform in a continuous mode, since various feed streams of reactants must be controlled, 2) using hazardous hydrogen halides (e.g., HF and HCl) as reactants, and 3) yielding products which may create a disposal problem.
U.S. Pat. No. 3,696,156 describes a method of removing perfluoroolefin and perfluorochloroolefin impurities from saturated fluoroperhalocarbon compounds having two to six carbon atoms, by contacting the impure fluoroperhalocarbon in the vapor phase at about 180 to 250 degrees C with alumina containing a basic alkali metal or alkaline earth metal hydroxide or oxide.
U.S. Pat. No. 5,233,107 describes a process for removing olefinic impurities from hydrogen-containing chlorofluorocarbons in the gas phase at 200 to 400 degrees C over a zeolite. The contaminated higher boiling chlorofluorocarbons are preheated to convert the liquid to the gas phase in advance. The addition of 0.5 to 10% air or oxygen by volume to the process stream is recommended to keep coking at a very low level.
One of the disadvantages of processes utilizing elevated temperatures is that they require handling hot gases contaminated with hazardous compounds. In addition, certain fluorocarbons are unstable and generate a variety of olefinic and aliphatic impurities at elevated temperatures especially in the presence of catalytic surfaces.
Hall et al. Chemistry and Industry, March 6, 1989, 145-146, describe activated carbon filters to provide protection against exposure to PFIB and note that some of the PFIB is hydrolysed to produce 2H-perfluoroisobutyric acid and hydrogen fluoride. After storage and reuse of the exposed filter, 1,1,3,3,3-pentafluoropropene and 1,1,1,3,3,3-hexafluoropropane were found in the effluent stream.
A system and method for purifying saturated fluoroperhalocarbon liquids by removing olefinic impurities, such as PFIB, therefrom have been disclosed in U.S. Pat. Nos. 5,300,714 and 5,507,941. Inorganic oxide, hydroxide, carbonate, or phosphate particles are used in the method.
England et al., J. Fluorine Chem. 1981, 17, 265-288, describe reactions of amines with a dimer of hexafluoropropene and a perfluorovinyl sulfide prepared from hexafluoropropene. Anhydrous ammonia was added to a solution of hexafluoropropene dimer to form (1-amino-2,2,3,3,3-pentafluoropropylidene)propanedinitrile.
Coffman et al., J. Org. Chem., 1949, 14, 747-753, reported that ammonia reacted with tetrafluoroethylene forms an amine which splits out HF to form difluoroacetonitrile which then forms a trimer.
An organic amine-impregnated activated carbon composition, which preferably has been pre-treated, has been used in breathing gas filters to enhance removal of various toxic perfluorocarbons as is disclosed in U.S. Pat. No. 5,462,908. There is no disclosure as to the composition of the treated material or the nature of the nucleophile used to form a stable immobilized adduct with fluoroalkenes.
An exhaustive review of one of the fluoroalkenes is presented in xe2x80x9cThe Chemistry of Perfluoroisobutene,xe2x80x9d by Y. V. Zeifman, et al., Russian Chemical Reviews, 1984, 53 (March), 256-273. Reactions of PFIB with numerous N, O, S, and P nucleophiles are discussed, without reference to their quantitative analytical application or the ability of these nucleophiles to react with other fluoroalkenes.
Briefly, this invention provides a method for removing one or more highly fluorinated or perfluorinated alkenes (also referred to hereinbelow as a xe2x80x9cfluoroalkenexe2x80x9d) from a fluid, comprising the step of contacting the fluid with ammonium hydroxide or an organic nitrogen-, sulfur-, or phosphorus-containing nucleophile wherein the nucleophilic atom is N, S, or P (hereinafter sometimes referred to as N-, S-, or P-nucleophiles) for a time sufficient to form a nitrogen-, sulfur-, or phosphorus-containing nucleophile-fluoroalkene adduct. Preferably, the N-, S-, or P-nucleophile is sorbed on, or coated on, or bonded to, or itself can be, a support and can be used in a bed or cartridge. More preferably, the N-, S-, or P-nucleophile which can be sorbed on, or coated on, or bonded to a support is enmeshed in or forms a fibrous matrix, preferably a nonwoven matrix, which provides an essentially homogeneous, porous material. Alternatively the N-, S-, or P-nucleophile can be sorbed on, coated onto, or bonded directly to a porous matrix. Preferably, the nucleophile is ammonia or it comprises a nitrogen, sulfur or phosphorus nucleophile compound.
The optimum amount of nucleophile that can be loaded on a support varies with the nature of the support. In general, it is preferred to load the nucleophile in an amount in the range of 0.1 to 10 weight percent, more preferably 0.1 to 5 weight percent, based on the weight of the support.
In another aspect, the present invention provides a method for quantifying a highly fluorinated or perfluorinated alkene, for example PFIB, comprising the steps of
a) providing a stable or unstable N-, S-, or P-containing nucleophile-fluoroalkene adduct, which may be sorbed to a support, and
b) quantifying for highly fluorinated or perfluorinated alkenes in a fluid by either
1) measuring highly fluorinated or perfluorinated alkenes when the N-, S-, or P-containing nucleophile-fluoroalkene adduct is stable, displacing the adduct from the support when necessary, or
2) measuring the fluoride ion for indirect quantification of highly fluorinated or perfluorinated alkenes, when the N-, S-, or P-containing nucleophile-fluoroalkene is unstable and produces fluoride ion.
In yet another aspect, the present invention describes a method for preparing an adduct comprising the step of contacting an immobilized N-, S-, or P-containing nucleophile with a fluid comprising a fluoroalkene for a time sufficient to form an N-, S-, or P-containing nucleophile-fluoroalkene adduct.
In still another aspect, the present invention provides a reactive particulate comprising an N-, S-, or P-containing nucleophile bonded or sorbed to a support which preferably has a high surface area. The reactive particulate can react with a highly fluorinated or perfluorinated alkene to produce an N-, S-, or P-containing nucleophile-fluoroalkene adduct.
In a further aspect, the present invention provides N-, S-, or P- containing nucleophile-fluoroalkene adducts which have eliminated HF to form new nucleophile-substituted olefinic compounds and which can be made to further react with other nucleophiles such as water or an alcohol to form a new secondary adduct which undergoes further reactions to form an amide or a vinyl ether when a nitrogen nucleophile was used to prepare the initial nucleophile-fluoroalkene adduct.
In a still further aspect, there are provided stable fluoroalkene-nucleophile derivatives which can be used as analytical standards for quantification and identification of the more toxic fluoroalkene species, reducing the difficulties of manufacture and shipping hazardous material.
As used in this application:
xe2x80x9cAdductxe2x80x9d means the addition product of a nucleophile and a fluoroalkene with or without the elimination of a byproduct;
xe2x80x9cfluidxe2x80x9d refers to a material that is either a liquid or a gas at 25xc2x0 C. and 760 mm Hg pressure, i.e., standard conditions;
xe2x80x9cfluoroalkenexe2x80x9d and xe2x80x9cfluoroolefinxe2x80x9d are used interchangeably;
xe2x80x9chighly fluorinated alkenexe2x80x9d means that more than half of the hydrogen atoms on the alkene have been replaced with fluorine atoms. It is preferred that the carbon atoms immediately adjacent to the unsaturated carbon-carbon bond of xe2x80x9chighly fluorinated alkenesxe2x80x9d will have more than half the total number of hydrogen atoms directly bonded to them replaced with fluorine atoms, and most preferably the highly fluorinated alkene is a perfluorinated alkene;
xe2x80x9chighly fluorinated alkyl groupxe2x80x9d will have more than half the total number of hydrogen atoms replaced with fluorine atoms. Although hydrogen atoms may remain, it is preferred that all hydrogen atoms be replaced with fluorine to form a perfluoroalkyl group;
xe2x80x9cstablexe2x80x9d means that the fluoroalkene-nucleophile adduct can survive the thermal stress it is subjected to when injected into a gas chromatographic column at elevated temperatures needed to elute, identify, and quantify the compounds of interest; and
xe2x80x9cnitrogen-, sulfur-, or phosphorus-nucleophilexe2x80x9d (xe2x80x9cN-, S-, or P-nucleophilexe2x80x9d) means any nucleophile comprising one or more of nitrogen, sulfur, or phosphorus as the nucleophilic site, and includes ammonia and any organic nucleophiles comprising one or more of nitrogen, sulfur, or phosphorus as the nucleophilic site.
A fluoroalkene of particular interest for quantifying in or removal from fluids is PFIB, which, because of its very toxic nature, is not available commercially for analytical instrument calibration. 2-Perfluorobutene, which is readily available (Lancaster Chemicals Inc., Lancaster, Pa.), can be chosen as a surrogate of close structure analogous to PFIB for calibration purposes in the present invention.
In a still further aspect, there is provided a porous, preferably non-woven, fibrous matrix, preferably a sheet material, comprising an immobilized reactive or sorptive N-, S-, or P-containing nucleophile coated on high surface area particulate entrapped in the fibrous sheet material, that upon reaction with a fluoroalkene produces a nucleophile-fluoroalkene adduct. The high surface area and close proximity of the entrapped sorptive particles results in rapid reaction kinetics due to the minimal diffusion distances between particles. Porous non-woven fibrous matrices useful in the present invention for immobilizing nucleophile-coated particles are disclosed, for example, in U.S. Pat. No. 5,635,060, which is incorporated herein by reference.
The present invention provides advantages in removing and quantifying the levels of fluoroalkenes in fluids. The quantitative method of the present invention permits positive identification of individual highly fluorinated or perfluorinated alkenes with a thousand-fold increase in sensitivity of quantitative measurements compared with existing gas chromatographic methods. In order to avoid contamination of fluids having high vapor pressure by generation of a toxic volatile highly fluorinated or perfluorinated alkene therein, the present invention provides a relatively nonvolatile adduct for removing such potential contaminants.
The present invention method is also advantageous in that stability of the adduct can be selected. In some embodiments, it may be desirable to produce an unstable adduct so as to decompose or remove highly fluorinated or perfluorinated alkenes. Hydrous ammonia (ammonium hydroxide), for example, forms an unstable series of adducts which release HF and appears to completely de-fluorinate PFIB to form tricyanomethane with the release of 8 fluoride ions. This is in contrast to England et al., supra, where anhydrous ammonia, when reacted with hexafluoropropylene dimer, formed a dicyanofluorocarbon, (1-amino-2,2,3,3,3-pentafluoropropylidene)propanedinitrile.
A sorbent can be used to sorb the reaction products. For example, molecular sieves such as Silicalite(trademark), a hydrophobic zeolite (UOP, Tarrytown, N.Y.), can be useful to sorb hydrogen fluoride (HF) and trap other low molecular weight reaction products, both of which can be produced when highly fluorinated or perfluorinated alkenes react with certain nucleophiles. In the present invention, Silicalite can act as a substrate for immobilizing the nucleophiles of the present invention and as a reactant and sorbent for reaction products. It is a preferred substrate in that it is not deactivated by water because of its six Angstrom pore size which prevents water molecule clusters from entering the high surface area interior of the particle.
In the present invention heating is not required to activate the nucleophiles. The present invention method takes place at room temperature (20-25 degrees C) and no pre-treatment of the nucleophile, as in U.S. Pat. No. 5,462,908, is required.
Highly fluorinated or perfluorinated alkenes that can be removed, immobilized, or quantified according to the method of the invention include alkenes of the formula 
wherein A=F or Rf; X=H, F, Cl, Rf, or YRf; Z=H, F, Cl, or Rf; and Y=O, N, or S; with the proviso that at least one A=F, and at most one of Z and X is H; wherein each Rf is independently selected from the group consisting of highly fluorinated or perfluorinated alkyl groups; and a combination of any two Rf groups can be linked to form a cyclic structure.
The Rf alkyl chains may contain from 1-20 carbon atoms, with 1-12 carbon atoms preferred. The Rf alkyl chains may be straight, branched, or cyclic. Up to twenty heteroatoms or radicals such as divalent oxygen, trivalent nitrogen or hexavalent sulfur may interrupt the skeletal chain, as is well recognized in the art. When Rf is or contains a cyclic structure, such structure preferably has 5 or 6 ring members, 1 or 2 of which can be heteroatoms. xe2x80x9cHighly fluorinatedxe2x80x9d means that the degree of fluorination on the chain is sufficient to provide the chain with properties, particularly electronic properties, similar to those of a perfluorinated chain. More particularly, a highly fluorinated alkyl group will have more than half the total number of hydrogen atoms replaced with fluorine atoms. Although hydrogen atoms may remain, it is preferred that at least two hydrogens on each carbon attached to the vinylic carbon atoms be replaced by fluorine, and it is more preferred that all hydrogen atoms be replaced with fluorine to form a perfluoroalkyl group. It is more preferred that at least two out of three hydrogens on the alkyl group be replaced with fluorine, still more preferred that at least three of four hydrogen atoms be replaced with fluorine and most preferred that all hydrogen atoms be replaced with fluorine to form a perfluorinated alkyl group. Preferably, Rf is CF3, C2F5 or C3F7.
The ability of highly fluorinated and perfluorinated alkenes to react with nuclcophiles is known (see, for example, Zeifman, supra), and is believed to be caused by the high electrophilicity of the carbon-carbon double bond, which, in turn, is believed to be due to the strong electron withdrawing effects of fluorine atoms and Rf groups, particularly CF3 groups, attached thereto, as well as the capacity of vinylic fluorine atoms to effectively conjugate with the carbon-carbon double bond. Of the known fluoroalkenes, perfluoroisobutene (PFIB), having the chemical formula 
can be regarded as the most reactive towards nucleophiles, which Zeifman (supra) attributed to the highly electrophilic character of the carbon-carbon double bond, the lability of the fluorine atoms of the CF3 group, and the possibility of further reactions of any nucleophile addition products due to the high CH-acidity of, e.g., a (CF3)2CHxe2x80x94CF2-group that may be formed by such an addition reaction.
Nucleophiles capable of reacting with highly fluorinated and perfluorinated alkenes include those comprising a nitrogen-, sulfur- or phosphorus nucleophilic site. Nitrogen-containing nucleophiles useful in the present invention include ammonia, primary amines, and secondary amines, preferably secondary amines. Ammonia is known to react with, for example, PFIB, to yield hexafluoroisobutyronitrile and hydrogen fluoride (Zeifman, supra).
Primary amines useful in the present invention include, in principle, all known primary amines, including aliphatic, aromatic, saturated and unsaturated heterocyclic primary amines. Although primary amines are not the preferred amine nucleophile of the present invention, liquid primary amines that can be easily and safely handled under ambient conditions (e.g. 25xc2x0 C. and one atmosphere pressure), and that readily adsorb or absorb onto a substrate or carrier, as described below, can be suitable nucleophiles for the present invention. Non-limiting examples of such primary amines include propyl amine, n-butyl amine, ethylenediamine, n-octyl amine, aniline, cyclohexylamine, benzylamine, and a myriad of amines known in the art.
Secondary amines are preferred in the present invention, and, in principle, can include all known secondary amines, including aliphatic, aromatic, saturated and unsaturated heterocyclic secondary amines. Secondary amines are preferred in the present invention because they can be capable of a 1:1 molar reaction with a highly fluorinated or perfluorinated alkene whereby any further reaction of the amine-fluoroalkene adduct with the secondary amine is unlikely, such that cleaner, more homogeneous reaction products can be produced. Non-limiting examples of secondary amines useful in the invention include dimethylamine, diethylamine, dibutylamine, dibenzylamine, morpholine, N-methyl aniline, methylbenzylamine, piperidine, pyrrole, pyrrolidine, pyrrolidinone, and piperazine. Preferably, secondary amines useful in the present invention include heterocyclic compounds, non-limiting examples of which include morpholine, piperidine, pyrrole, and pyrrolidine. Heterocyclic secondary amines are preferred because of their ease of handling and their ready reactivity. Morpholine is the most preferred secondary amine, for purposes of the present invention.
As noted above, the lability of fluorine atoms in the reaction of amines with fluoroalkenes can generate hydrogen fluoride, HF. Therefore, the reaction can be driven to completion by effective removal of HF. For this reason, amines are the preferred nucleophile for the present invention, since essentially all known amines exhibit sufficient basicity to scavenge the HF as it is formed, so that the reaction is driven to completion. As described in more detail below, the generation and quantification of HF in these reactions provides a useful means of measuring the amount of fluoroalkene in a fluid.
Sulfur-containing nucleophiles useful in the invention comprise those that are capable of forming a sulfide anion upon reaction with a base. Examples include thiophenol, benzyl mercaptan, allyl mercaptan, and salts thereof, such as mercaptides and thiophenoxides. The reaction of fluoroalkenes with sulfur nucleophiles can produce either HF or MF, where M is a metal such as sodium, potassium, or the like, that forms the mercaptide or thiophenoxide salt, and disubstitution to replace both vinylic fluorine atoms can take place readily.
Phosphorus-containing nucleophiles useful in the invention can include trialkyl phosphites (e.g., trimethyl phosphite, triethyl phosphite, tri-n-propyl phosphite, tri-n-butyl phosphite), dialkyl phosphines (e.g., di-isopropyl phosphine), and trialkyl phosphines (e.g., trimethyl phosphine, triethyl phosphine, tri-n-propyl phosphine, tri-n-butyl phosphine).
Particulate supports to which nucleophiles of the invention can be bonded or upon which nucleophiles can be sorbed include those that are substantially insoluble in any aqueous or organic fluids comprising fluoroalkenes that are to be adsorbed, remediated, or quantified by methods of the invention. xe2x80x9cOrganic fluidsxe2x80x9d includes highly fluorinated or perfluorinated working fluids known in commerce, such as refrigerants, thermal management fluids, dielectric fluids, and the like. The particle can be an organic polymer, for example, poly(divinylbenzene), poly(styrene-co-divinylbenzene), or poly- or copoly-(meth)acrylic acid esters, and derivatives thereof; inorganic oxide particles, for example, silica, alumina, titania, zirconia and other ceramic materials, to which optionally organic groups can be bonded or coated, or which can be coated with aqueous- or organic-insoluble, non-swellable sorbent material or the surface (internal and/or external) of which can be derivatized to provide a coating of insoluble, non-swellable sorbent material; carbon particles, particularly activated charcoal; molecular sieves, polymer-coated, carbon-clad inorganic oxide particles, such as those described in U.S. Pat. No. 5,271,833, and the like.
Particulate materials useful in the invention can have a diameter of from about 0.1 to about 600 micrometers, preferably in the range of 1-100 micrometers, more preferably in the range of 3 to 100 micrometers. Particle size is chosen so as to maximize available surface area while also optimizing fluid flow rate through the particle matrix. Preferably the surface area is in the range of 50 to 1000 meter2/gram or more. Particulate supports can have a regular shape, for example spherical, or an irregular shape. It has been found advantageous in some instances to use particulate materials in two or more particle size ranges falling within the broad range.
The insoluble, aqueous- or organic-insoluble, non-swellable sorbent coatings useful in the invention generally have a thickness in the range of one molecular layer to about 100 nanometers. Many particles having coated surfaces are known, including modified silica particle, for example, silica particle having bonded thereto organic groups, preferably cyanopropyl, cyclohexyl, phenyl, ethyl, butyl, octyl, and octadecyl groups. Such coatings preferably readily sorb nucleophiles of the invention and are inert toward them.
Sorptive coatings which can be applied to particulate materials can be either thin mechanical coatings of insoluble, non-swellable polymers such as crosslinked silicones, poly(butadienes), etc., or covalently bonded organic groups such as aliphatic groups of varying chain length (for example, from 2 to 18 carbon atoms).
Molecular sieves useful in the invention, which include zeolites, are inorganic, crystalline materials, usually aluminosilicate compositions, in which the crystal framework of aluminum and silicon atoms forms a three-dimensional network of internal cavities having a honeycomb-like structure. Many molecular sieves of varying size and constitution are commercially available.
A particularly useful molecular sieve is Silicalite inorganic hydrophobic zeolite commercially available from UOP, Tarrytown, N.Y., under the trade name Abscents(trademark).
A coconut-based activated charcoal particulate for use in the invention is preferred.
The particles of the invention can be enmeshed in various porous, fibrous, nonwoven webs or matrices. Types of webs or matrices include fibrillated-polytetrafluoroethylene (PTFE), microfibrous webs, macrofibrous webs, and polymer pulps. Alternatively, in use, particles can be held in a bed, including a fluidized bed, or packed in a column or tube. Nucleophilic materials can be coated or sorbed onto particulate prior to or after the particulate becomes enmeshed in a web or is placed in a bed or packed in a column.
Many types of fibrous, nonwoven webs or matrices can be useful in the invention, including;
1. Fibrillated PTFE
PTFE composite sheet material can be prepared by blending the particulate or combination of particulates employed with an aqueous PTFE emulsion until a uniform dispersion is obtained and adding a volume of process lubricant up to approximately one half the volume of the blended particulate. Blending takes place along with sufficient process lubricant to exceed sorptive capacity of the particles in order to generate the desired porosity level of the resultant article. Preferred process lubricant amounts are in the range of 3 to 200 percent by weight in excess of that required to saturate the particulate, as is disclosed in U.S. Pat. No. 5,071,610. The aqueous PTFE dispersion is then blended with the particulate mixture to form a mass having a putty-like or dough-like consistency. The sorptive capacity of the solids of the mixture is noted to have been exceeded when small amounts of water can no longer be incorporated into the mass without separation. This condition should be maintained throughout the entire mixing operation. The putty-like mass is then subjected to intensive mixing at a temperature and for a time sufficient to cause initial fibrillation of the PTFE particles. Preferably, the temperature of intensive mixing is up to 90xc2x0 C., preferably it is in the range of 0xc2x0 to 90xc2x0 C., more preferably 20xc2x0 to 60xc2x0 C.
Preferably, the weight ratio of particulate to PTFE is in the range of 40:1 to 1:40, more preferably 30:1 to 1:30, and most preferably in the range of 20:1 to 1:20.
Mixing times will typically vary from 0.2 to 2 minutes to obtain the necessary initial fibrillation of the PTFE particles. Initial mixing causes partial disoriented fibrillation of a substantial portion of the PTFE particles.
Initial fibrillation generally will be noted to be at an optimum within 60 seconds after the point when all components have been fully incorporated into a putty-like (dough-like) consistency.
Devices employed for obtaining the necessary intensive mixing are commercially available intensive mixing devices which are sometimes referred to as internal mixers, kneading mixers, double-blade batch mixers as well as intensive mixers and twin screw compounding mixers. The most popular mixer of this type is the sigma-blade or sigma-arm mixer. Some commercially available mixers of this type are those sold under the common designations Banbury mixer, Mogul mixer, C. W. Brabender Prep mixer and C. W. Brabender sigma blade mixer. Other suitable intensive mixing devices may also be used.
The soft putty-like mass is then transferred to a calendering device where the mass is calendered between gaps in calendering rolls preferably maintained at a temperature up to 125xc2x0 C., preferably in the range of 0xc2x0 to about 100xc2x0 C., more preferably in the range of 20xc2x0 C. to 60xc2x0 C., to cause additional fibrillation of the PTFE particles of the mass, and consolidation while maintaining the water level of the mass at least at a level of near the sorptive capacity of the solids, until sufficient fibrillation occurs to produce the desired nonwoven web. Preferably the calendering rolls are made of a rigid material such as steel. A useful calendering device has a pair of rotatable opposed calendering rolls each of which may be heated and one of which may be adjusted toward the other to reduce the gap or nip between the two. Typically, the gap is adjusted to a setting of 10 millimeters for the initial pass of the mass and, as calendering operations progress, the gap is reduced until adequate consolidation occurs. At the end of the initial calendering operation, the resultant sheet is folded and then rotated 90xc2x0 to obtain biaxial fibrillation of the PTFE particles. Smaller rotational angles (e.g., 20xc2x0 to less than 90xc2x0) may be preferred in some extraction and chromatographic applications to reduce calender biasing, i.e., unidirectional fibrillation and orientation. Excessive calendering (generally more than two times) reduces the porosity which in turn reduces the flow-through rate.
During calendering, the lubricant level of the mass is maintained at least at a level of exceeding the absorptive capacity of the solids by at least 3 percent by weight, until sufficient fibrillation occurs and to produce porosity or void volume of at least 30 percent and preferably 40 to 70 percent of total volume. Increased lubricant results in increased pore size and increased total pore volume as is disclosed in U.S. Pat. No. 5,071,610.
The calendered sheet is then dried under conditions which promote rapid drying yet will not cause damage to the composite sheet or any constituent therein. Preferably drying is carried out at a temperature below 200xc2x0 C. The preferred means of drying is by use of a forced air oven. The preferred drying temperature range is from 20xc2x0 C. to about 70xc2x0 C. The most convenient drying method involves suspending the composite sheet at room temperature for at least 24 hours. The time for drying may vary depending upon the particular composition, some particulate materials having a tendency to retain water more than others.
The resultant composite sheet preferably has a tensile strength when measured by a suitable tensile testing device such as an Instron (Canton, Mass.) tensile testing device of at least 0.5 MPa. The resulting composite sheet has uniform porosity and a void volume of at least 30 percent of total volume.
The PTFE aqueous dispersion employed in producing the PTFE composite sheet usefull in this invention is a milky-white aqueous suspension of minute PTFE particles. Typically, the PTFE aqueous dispersion will contain about 30 percent to about 70 percent by weight solids, the major portion of such solids being PTFE particles having a particle size in the range of about 0.05 to about 0.5 micrometers. The commercially available PTFE aqueous dispersion may contain other ingredients, for example, surfactant materials and stabilizers which promote continued suspension of the PTFE particles.
Such PTFE aqueous dispersions are presently commercially available from Dupont de Nemours Chemical Corp. (Wilmington, Del.), for example, under the trade names Teflon(trademark) 30, Teflon(trademark) 30B or Teflon(trademark) 42. Teflon(trademark) 30 and Teflon(trademark) 30B contain about 60 percent solids by weight which are for the most part 0.05 to 0.5 micrometer PTFE particles and from about 5.5 percent to about 6.5 percent by weight (based on weight of PTFE resin) of non-ionic wetting agent, typically octylphenol polyoxyethylene or nonylphenol polyoxyethylene. Teflon 42 contains about 32 to 35 percent by weight solids and no wetting agent but has a surface layer of organic solvent to prevent evaporation. A preferred source of PTFE is FLUONM, available from ICI Americas, Inc. Wilmington, Del.
In other embodiments of the present invention, the fibrous membrane (web) can comprise non-woven, macro- or microfibers preferably selected from the group of fibers consisting of polyamide, polyolefin, polyester, polyurethane, glass fiber, polyvinylhalide, or a combination thereof. The fibers preferably are polymeric. (If a combination of polymers is used, a bicomponent fiber may be obtained.) If polyvinylhalide is used, it preferably comprises fluorine of at most 75 percent (by weight) and more preferably of at most 65 percent (by weight). Addition of a surfactant to such webs may be desirable to increase the wettability of the component fibers.
2. Macrofibers
The web can comprise thermoplastic, melt-extruded, large-diameter fibers which have been mechanically-calendered, air-laid, or spunbonded. These fibers have average diameters in the general range of 50 xcexcm to 1,000 xcexcm.
Such non-woven webs with large-diameter fibers can be prepared by a spunbond process which is well known in the art. (See, e.g., U.S. Pat. Nos. 3,338,992, 3,509,009, and 3,528,129.) As described in these references, a post-fiber spinning web-consolidation step (i.e., calendering) is required to produce a self-supporting web. Spunbonded webs are commercially available from, for example, AMOCO, Inc. (Napierville, Ill.).
Non-woven webs made from large-diameter staple fibers can also be formed on carding or air-laid machines (such as a Rando-Webber(trademark) Model 12BS made by Curlator Corp., East Rochester, N.Y.), as is well known in the art. See, e.g., U.S. Pat. Nos. 4,437,271, 4,893,439, 5,030,496, and 5,082,720.
A binder is normally used to produce self-supporting webs prepared by the air-laying and carding processes and is optional where the spunbond process is used. Such binders can take the form of resin systems which are applied after web formation or of binder fibers which are incorporated into the web during the air laying process.
Examples of common binder fibers include adhesive-only type fibers such as Kodel(trademark) 43UD (Eastman Chemical Products, Kingsport, Tenn.) and bicomponent fibers, which are available in either side-by-side form (e.g., Chisso ES Fibers, Chisso Corp., Osaka, Japan) or sheath-core form (e.g., Melty(trademark) Fiber Type 4080, Unitika Ltd., Osaka, Japan). Application of heat and/or radiation to the web xe2x80x9ccuresxe2x80x9d either type of binder system and consolidates the web.
Generally speaking, non-woven webs comprising macrofibers have relatively large voids. Therefore, such webs have low capture efficiency of small-diameter particulate (reactive supports) which is introduced into the web. Nevertheless, particulate can be incorporated into the non-woven webs by at least four means. First, where relatively large particulate is to be used, it can be added directly to the web, which is then calendered to actually enmesh the particulate in the web (much like the PTFE webs described previously). Second, particulate can be incorporated into the primary binder system (discussed above) which is applied to the non-woven web. Curing of this binder adhesively attaches the particulate to the web. Third, a secondary binder system can be introduced into the web. Once the particulate is added to the web, the secondary binder is cured (independent of the primary system) to adhesively incorporate the particulate into the web. Fourth, where a binder fiber has been introduced into the web during the air laying or carding process, such a fiber can be heated above its softening temperature. This adhesively captures particulate which is introduced into the web. Of these methods involving non-PTFE macrofibers, those using a binder system are generally the most effective in capturing particulate. Adhesive levels which will promote point contact adhesion are preferred.
Once the particles have been added, the loaded webs are typically further consolidated by, for example, a calendering process. This further enmeshes the particles within the web structure.
Webs comprising larger diameter fibers (i.e., fibers which average diameters between 50 xcexcm and 1,000 xcexcm) have relatively high flow rates because they have a relatively large mean void size.
3. Microfibers
When the fibrous web comprises non-woven microfibers, those microfibers provide thermoplastic, melt-blown polymeric materials having active particles dispersed therein. Preferred polymeric materials include such polyolefins as polypropylene and polyethylene, preferably further comprising a surfactant, as described in, for example, U.S. Pat. No. 4,933,229. Alternatively, surfactant can be applied to a blown microfibrous (BMF) web subsequent to web formation. Polyamide can also be used. Particulate can be incorporated into BMF webs as described in U.S. Pat. No. 3,971,373.
Microfibrous webs of the present invention have average fiber diameters up to 50 xcexcm, preferably from 2 xcexcm to 25 xcexcm, and most preferably from 3 xcexcm to 10 xcexcm. Because the void sizes in such webs range from 0.1 xcexcm to 10 xcexcm, preferably from 0.5 xcexcm to 5 xcexcm, flow through these webs is not as great as is flow through the macrofibrous webs described above.
4. Cast Porous Membranes
Solution-cast porous membranes can be provided by methods known in the art. Such polymeric porous membranes can be, for example, polyolefin including polypropylene, polyamide, polyester, polyvinyl chloride, and polyvinyl acetate fibers.
5. Fibrous Pulps
The present invention also provides the use of a sheet material comprising a porous fibrous pulp, preferably a polymeric pulp, comprising a plurality of fibers that mechanically entrap active particles, and preferably the sheet also comprises a polymeric hydrocarbon binder, the weight ratio of particles to binder being at least 13:1 and the ratio of average uncalendered sheet thickness to effective average particle diameter being at least 125:1.
Generally, the fibers that make up the porous polymeric pulp of the sheet material useful in the present invention can be any pulpable fiber (i.e., any fiber that can be made into a porous pulp). Preferred fibers are those that are stable to radiation and/or to a wide range of pH (1 through 14). Examples include polyamide fibers and those polyolefin fibers that can be formed into a pulp including, but not limited to, polyethylene and polypropylene. Particularly preferred fibers are aromatic polyamide fibers and aramid fibers because of their stability to both radiation and highly caustic fluids. Examples of useful aromatic polyamide fibers are those fibers of the nylon family. Polyacrylic nitrile, cellulose, and glass can also be used. Combinations of pulps can be used.
Examples of useful aramid fibers are those fibers sold under the trade name Kevlar(trademark) (DuPont, Wilmington, Del.). Kevlar fiber pulps are commercially available in three grades based on the length of the fibers that make up the pulp. Regardless of the type of fiber(s) chosen to make up the pulp, the relative amount of fiber in the resulting sheet (when dried) ranges from about 12.5 percent to about 30 percent (by weight), preferably from about 15 percent to 25 percent (by weight).
Useful binders for sheets useful in the present invention are those materials that are stable over a range of pH (especially high pH) and that exhibit little or no interaction (i.e., chemical reaction) with any of the fibers of the pulp, the particles entrapped therein, or fluoroalkenes. Polymeric hydrocarbon materials, originally in the form of latexes, have been found to be especially useful. Common examples of useful binders include, but are not limited to, natural rubbers, neoprene, styrene-butadiene copolymer, acrylate resins, and polyvinyl acetate. Preferred binders include neoprene and styrene-butadiene copolymer. Regardless of the type of binder used, the relative amount of binder in the resulting sheet (when dried) is about 3 percent to about 7 percent, preferably about 5 percent. The preferred amount has been found to provide sheets with nearly the same physical integrity a sheets that include about 7 percent binder while allowing for as great a particle loading as possible. It may be desirable to add a surfactant to the fibrous pulp, preferably in small amounts up to about 0.25 weight percent of the composite.
Because the capacity and efficiency of the sheet depends on the amount of particles included therein, high particle loading is desirable. The relative amount of particles in a given sheet material useful in the present invention is preferably at least about 65 percent (by weight), more preferably at least about 70 percent (by weight), and most preferably at least about 75 percent (by weight). Additionally, the weight percentage of particles in the resulting sheet is at least 13 times greater than the weight percentage of binder, preferably at least 14 times greater than the weight percentage of binder, more preferably at least 15 times greater than the weight percentage of binder.
Regardless of the type or amount of the particles used in the sheet material useful in the present invention, they are mechanically entrapped or entangled in the fibers of the porous pulp. In other words, the particles are not covalently bonded to the fibers.
The method of reacting a nucleophile, particularly an amine or a nitrogen-containing nucleophile, with a fluoroalkene as described herein finds utility in absorbing and removing the fluoroalkenes from fluids in which they may be present. In the invention, xe2x80x9cremovalxe2x80x9d includes chemical reactions that destroy or sufficiently change or reduce the volatility or toxicity of fluoroalkenes. Such chemical reactions may liberate hydrogen fluoride (HF), such that determination of the amount of HF liberated provides an indirect measure of the amount of fluoroalkene in the fluid. Thus, the method of the invention is useful for remediation of highly fluorinated or perfluorinated working fluids, including industrial reaction and waste streams, to remove potentially toxic or dangerous fluoroalkenes, the most notable of which is PFIB. In addition, the method of the invention provides a convenient, direct method for quantification of certain fluoroalkenes due to precise identification of fluoroalkene-nucleophile adduct(s) by, for example, GC/MS methods. Finally, the method of the invention provides a means of continuous cleaning of highly fluorinated and perfluorinated working fluids by circulating them through a bed or column or particle-loaded fibrous web comprising support particles to which or on which nucleophiles are sorbed or coated.
In a preferred embodiment of the invention, an amine, preferably a secondary amine, and more preferably morpholine, was adsorbed onto silica gel particles that previously had been packed into a tube. A fixed amount of fluid containing a fluoroalkene was passed through the tube, and the effluent was analyzed by GC/MS for the presence of the fluoroalkene to determine the capacity of the tube.
Alternatively, particles, such as activated charcoal, can be enmeshed in a fibrillated PTFE web, such as are commercially available as Empore(trademark) nonwoven webs, from 3M, St. Paul, Minn. Preferably, the activated charcoal particles are coated with morpholine by soaking the particle-loaded Empore(trademark) web in morpholine solution, then air-drying the web to remove residual solvent. Then the fluid containing a fluoroalkene to be quantified or removed is passed through the morpholine treated web which is placed in a standard solid phase extraction (SPE) web holder.
Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.
Perfluorinated liquids can be used as coolants for electronic devices (e.g., supercomputers) and as heat transfer media in vapor-phase soldering processes. Toxic fluoroalkene impurities can form upon transient heating of perfluorinated liquids which are used in many industrial processes. The methods and materials of the present invention are environmentally advantageous because (1) they can be used to remove fluoroalkenes, including PFIB, from fluids frequently used in industry and (2) they teach methods to detect and monitor the presence of these reactive fluoroalkenes at low levels.